Consider postponing processing of PLC files

[RobertoRoos]

Now all sources configured in conf.py are loaded from the start and put into a list to be searched in later.
This could be replaced by only indexing the files at the start and only loading them when they are called from an auto... method.

This might make plcdoc more robust and quick as no unnecessary processing is done.

[RobertoRoos]

One problem: an object has to be correlated with a file name, but only the declaration inside the files are leading. So the XML should be already be processed too.

EDIT: Actually, Visual Studio will automatically rename files to match with their internal declaration, so only relying on the file name would be fine.
To my knowledge there are never multiple declarations for a single file.

EDIT2: Visual Studio gives an error trying to open a file with more than one declaration. And TwinCAT fails to compile it.